I don’t know about the stock in other countries, but in mine (India), while the Visual Novels themselves are not for sale, the mangas are all up for sale on amazon.in, and quite a lot of price ends up going in shipping as well as the cost of the mangas itself, to the point where I have to evaluate the worth of the mangas itself and which one I absolutely must have. I guess that’s a sign of localisation?
Although, the same problem persists here: all volumes up until the last volume of Dawn of the Golden Witch are available, if not sold out (and very few are sold out, maybe one in Turn and Alliance), Requiem and Twilight are virtually unheard of, let alone Umineko Hane. So yes, I think they’ve either hit a wall or they’ve stopped bothering. Or ours is simply a country where sales hasn’t exactly been booming for Umineko on account of the few people who know that it even exists.